For the sake of resource saving and efficiency promotion, there has recently been an increase demand for image forming devices which can print images on both sides of a sheet, and image forming systems in which the image forming device is combined with peripheral devices so that image forming is efficiently carried out. Such a image forming system as above adopts switchback mechanism, etc. and hence a sheet on which an image has already been formed on one side is reversed and fed to the image forming device again, and an image is formed on the other side of the sheet. Further, the image forming system sends a sheet with finished images to post-treatment devices, in order to subject the sheet to a punch hole treatment for making a punch hole and a staple treatment for stapling.
On the occasion of image forming by which images are formed on both sides of a sheet (hereinafter, this image forming process will be referred to as duplex image forming) or on the occasion of carrying out the foregoing post-treatments, sheets which are successively fed are not stacked in proper alignment, since the sheets are fed over long distances. In particular, the misalignment tends to occur in the direction orthogonal to the direction of sheet feeding. For this reason, in the case of the duplex image forming, an image on one side is not properly aligned with an image on the other side so that the results of the image forming have insufficient quality. Further, the above-mentioned post-treatment results in production of a book in which sheets are not properly aligned, and hence it is not possible to provide high-quality books. Moreover, when sheets are fed over great distances, the misalignment could cause damage to the sheets.
To solve this problem, an operation of aligning a sheet (hereinafter, this operation will be referred to as alignment operation) is carried out before feeding the sheet to an image forming device again and before carrying out post-treatments in a post-treatment device, and may be carried out in midstream of a sheet feeding path when the path is particularly long. This alignment operation aims at eliminating misalignment in the direction orthogonal to the direction of sheet feeding. More specifically, after the sheet pinched by pairs of feed rollers for feeding sheets is released, the misalignment of the sheet is corrected by an aligning device including alignment means called jogger. Then the pairs of feed rollers pinch the sheet again, and post-treatments are carried out. In this arrangement, the release of the sheet pinched by the pairs of feed rollers is carried out using a solenoid.
However, the above-mentioned conventional aligning device has such a problem that the release of the sheet requires strong force.
This is because the pinching forces exerted by the pairs of feed rollers are set so as to be strong in order to surely feed the sheet without causing misalignment. Since a plurality of pairs of feed rollers are provided at intervals allowing to feed a minimum-sized sheet, it is necessary to release the pinching forces exerted by the plurality of pairs all at once, when aligning operation for a maximum-sized sheet is carried out. For instance, the aligning operation for an A3-sized sheet requires to release the forces exerted by 3-4 pairs of feed rollers all at once. Further, to avoid complicated machinery, conventional aligning devices release pinching forces of all pairs of feed rollers irrespective of the size of sheet. This is also the reason why the release of the pinching forces requires strong power.
Since the release of the pinching forces requires strong power, the use of a solenoid which exerts strong power is necessary, and this causes the increase of costs.
Moreover, the foregoing aligning device separately includes a drive unit for the pairs of feed rollers and the solenoid so that upsizing of the device is unavoidable.